The present invention relates to a reactive power compensation apparatus and, more particularly, to a reactive power compensation apparatus for effectively compensating reactive power in a system for supplying power from an AC power supply system through an AC main line to a load subject to great variations in reactive power.
Now a days, large-capacity arc furnace equipment is often connected to and operated by an AC power supply system. Reactive power which abruptly varies according to a state of melt in an arc furnace, is induced at the power supply. The abruptly varying reactive power distorts a voltage waveform in association with an impedance of the power supply system, causing flickering of illumination equipment and degrading the utilization rate of power supply equipment. For this reason, if a large-capacity arc furnace or the like is installed, it is connected in parallel with a reactive power compensation apparatus. The compensation apparatus compensates for the abrupt variations in reactive power generated by the arc furnace.
A conventional reactive power compensation apparatus is described Power Conversion Techniques for Controlling Reactive Power and Harmonics, Ed. General Committee of Reactors, Technical Report from the Institute of Electricity (Volume II), No. 76, April, 1979, PP. 26-31. The system configuration of the power supply system is shown in FIG. 1.
Referring to FIG. 1, reference numeral 10 denotes a load such as an arc furnace. Steel scrap or the like is charged in furnace 10 and electrodes 11 are energized to heat and melt the steel scrap or the like. Reference numeral 9 denotes a furnace transformer.
Reactive power compensator 100 has reactor section 300, and phase advance capacitor section 200 serves as a harmonic filter. Reactor section 300 comprises reactors 302U to 302W, anti-parallel-connected thyristors 301U to 301W respectively connected in series with reactors 302U to 302W, load current transformers 81R, 81S, and 81T, voltage detection transformer 70, and its control circuit 350. Control circuit 350 detects the reactive power of furnace 10, and the firing angles of thyristors 301U to 301W are controlled according to the detected reactive power, thereby controlling currents supplied to the reactors. More specifically, compensator 100 cooperates with capacitor section 200, to control currents flowing through reactors 302U to 302W and to generate advanced reactive power equal to delayed reactive power generated by furnace 10. The advanced reactive power appears at lines 51R, 51S, and 51T. Therefore, the reactive power is cancelled at part 4 of a three-phase main line, and only effective power of the load flows through main line 4. The voltage distortion of main line 4 is surpressed, and the utilization efficiency of the power supply equipment can be improved by a degree caused by cancelling of the reactive power. Reference numerals 3 denote system impedances present in the three-phase AC power supply system; and 1, a power supply such as a three-phase AC power supply system or a three-phase main line.
With the above arrangement, accurate detection of the reactive power generated by load 10 in control circuit 350 in compensator 100 is the key to high performance of the apparatus. A typical example of a conventional reactive power detector is disclosed in Japanese Patent Disclosure (Kokai) No. 59-139416. In this circuit, product q of a 90.degree. lagged main line voltage and a load current is calculated. Product q includes a DC component (i.e., a reactive power component) and an AC component oscillating at a frequency twice the fundamental wave frequency. Signal q is filtered through a low-pass filter to detect the DC component thereof (representing the reactive power), and the current supplied to the reactor section is controlled on the basis of the DC component.
Other various reactive power detection methods have been proposed. However, the principle of these methods can be reduced to the one described in Japanese Patent Disclosure No. 59-139416.
The conventional reactive power compensation apparatus has been described. However, the conventional apparatus has the following drawback:
When power variations (including effective and reactive power components) in reactive power generated by an arc furnace are analyzed, these variations include a constant DC component (the in-phase power obtained by the in-phase voltage and current) and a variation component (the opposite phase power caused by the in-phase voltage and the opposite phase current). According to the conventional reactive power detection methods, there is no concept for separating the in-phase power from the opposite phase power. For this reason, the in-phase and opposite phase power components are mixed and the composite component is regarded as a simple variation. The reactor current is controlled by the above composite component. According to the conventional reactive power compensation apparatus, a compensation object is indeterminate. That is, it is impossible to discriminate if the in-phase reactive power (the constant component) or the opposite phase reactive power (the variation component) is controlled. As a result, there is no way to develop power control for higher performance.
A strong demand has recently arisen for improving quality of power in AC power systems. In order to satisfy this demand, a high-performance flicker-preventing reactive power compensation apparatus for an arc furnace or the like and high-quality control for stabilizing a reactive power compensation apparatus for an AC power system are required. For this purpose, a demand has arisen for a reactive power compensation apparatus employing a good power detection method (a method for detecting components including the effective and reactive components) based on a new and improved concept of control.